A method of making sludge precipitated in a chromium plating impurity recovery electrolysis tank harmless.
1. Field of the Invention
This invention relates to a method of making sludge comprising harmful substances produced in large quantities in plating works, harmless.
2. Description of the Prior Art
At present, methods for the treatment of chromium plating solutions and chromium plating effluent in plating works may be broadly divided into 3 types.
1. After reducing hexavalent chromium in chrome effluent to trivalent chromium using a reducing agent, it is precipitated as chromium hydroxide, the flow of supernatant liquid is controlled as process water, and discharged.
2. An ion exchange resin is used. Chromium effluent is subjected to ion exchange on a free base type, strongly basic anion exchange resin, and recovered as a tribasic chromic acid. In this ion exchange method, when the chromium effluent is treated by ion exchange, the upper concentration limit of chromic acids has to be controlled, and it is necessary to remove or insert the strongly basic anion exchange resin. After adjusting pH, hexavalent chromium is reduced to trivalent chromium, and a reagent is added to precipitate the chromium as chromium hydroxide which is separated.
3. In the impurity recovery electrolysis tank, chromium is re-used as chromium plating solution, and metal ions of impurities are precipitated as iron hydroxide.
The chromium hydroxide in the first and second methods, and the iron hydroxide in the third method, are harmful, toxic substances. At present, after reduction and neutralization, the sludge is treated so that it does not correspond to any of the toxic or dangerous substances specified in Treatment Regulations (Elution Landfill Regulations for Industrial Waste Containing Hexavalent Chromium Not Exceeding 1.5 mg/l), and is then buried.
The iron hydroxide of the third method is produced in large quantity when the chromium plating solution (chromium plating effluent which is periodically replaced) in the chrome treatment room is periodically collected for quality control purposes, and after filtering off solid suspended and floating matter contained therein, metal ions such as iron, copper, zinc and chromium contained in the chromium plating effluent are separated in the impurity recovery electrolysis tank, and the remaining solution is re-used as a plating solution. It is also produced in large quantity when the mixed chromium plating solution, chromium plating effluent and chromic acid wash water in the Closed Recycling System for Chromium Plating Solution, Chromium Plating Effluent or Chromic Acid Wash Water described in Japanese Patent Laid-Open No. Hei 9-165699 already proposed by the Inventor is collected, the solution is filtered, metal ions such as iron, copper, zinc and chromium contained in the mixed solution are separated in the impurity recovery electrolysis tank, and the remaining solution is re-used as a plating solution.
However, there is a basic problem in that, at present, the elution amount of hexavalent chromium can only be reduced to approximately the amount of 1.5 mg/l or less specified in the Waste Landfill Regulations.
In addition, there are major problems in the reducing and neutralizing processes in that equipment, treatment reagent and pollution treatment (sludge treatment) costs are high, and advanced operation and control technology is required which exceeds the management capacity of small and medium-sized industries.
In this context, the Inventor has proposed Japanese Patent Application No. Hei 8-102527 which offers a method of rendering sludge harmless.
In this previous application, sludge which has separated in an impurity recovery electrolysis tank is washed with water, the sludge which has sedimented after washing is dried, and it is then calcinated to make it harmless.
However, this previous application is intended to deal with only small amounts of sludge, i.e. 10 g, and there was no unexpected elution of hexavalent chromium. However, when 100 g or more of sludge was calcinated and examined, hexavalent chromium was eluted, so this method could not render sludge harmless with high reproducibility.
This invention, which was conceived in view of the aforesaid situation, therefore aims to resolve current problems in sludge treatment techniques, and render sludge containing harmful substances, which is produced in large quantity in plating works, harmless with high reproducibility by effectively using certain substances.
The inventor, as a result of extensive studies on sludge with adhering hexavalent chromium, discovered that natural organic substances could remove hexavalent chromium and thereby arrived at the present invention.
The essential feature of this invention is to remove sludge, obtained by separating metal ions such as iron, copper, zinc and chromium contained in chromium plating effluent or a mixture of chromium plating effluent and chromic acid wash water, wash the sludge with water, separate and retain the wash water containing chromium, dry the sludge which has sedimented after washing, add a natural organic substance to the sludge, and reduction calcinate it.
Preferably, the invention will provide an effective use for rice husks, a natural organic substance whereof 80% of the total production in Japan is treated as industrial waste.
The above techniques offer the following advantages.
According to this method of rendering sludge which has sedimented in a chromium plating impurity recovery electrolysis tank harmless, when a natural organic substance was added and stirred into the sludge with adhering hexavalent chromium and the product was reduction calcinated, iron oxide, ferrous oxide and trivalent chromium (Fe. Cr)2O3 were converted to solids in the form of a ferrite, and elution of hexavalent chromium was not detected.
The reason why hexavalent chromium is not detected is unclear, but it appears that the hexavalent chromium adhering to the sludge prior to calcination is converted to trivalent chromium during calcination by phenol polymers contained in the natural organic substance which acts as an organic reducing agent, and harmless complexes of iron oxide, ferrous oxide and trivalent chromium are then formed.